SSL devices can have different configurations for electrical contacts. For example, FIGS. 1A and 1B are cross-sectional and plan views of an SSL device 10 with lateral contacts. As shown in FIG. 1A, the SSL device 10 includes a substrate 12 carrying an LED structure 11 comprised of N-type gallium nitride (GaN) 14, GaN/indium gallium nitride (InGaN) multiple quantum wells (“MQWs”) 16, and P-type GaN 18. The SSL device 10 also includes a first contact 20 on the N-type GaN 14 and a second contact 22 on the P-type GaN 18. As shown in FIG. 1B, the first and second contacts 20 and 22 can include first and second contact fingers 20a, 20b, 22a, and 22b, respectively. FIGS. 2A and 2B are cross-sectional and plan views of another SSL device 10′ with vertical contacts. The first contact 20 includes a plurality of conductive fingers 21 (three are shown for illustration purposes) coupled to one another by a cross member 23. The second contact 22 (FIG. 2B) includes a reflective and conductive material (e.g., aluminum).
SSL devices with vertical contacts are often preferred because of their higher light extraction efficiencies, better thermal properties, and superior current spreading characteristics than those with lateral contacts. However, in the SSL device 10′ of FIGS. 2A and 2B, the N-type GaN 14 typically has a large thickness of about 4-6 μm to reduce the dislocation density in the N-type GaN 14 to an acceptable level. The large thickness may also be required for adequate current spreading through the N-type GaN 14 and for roughening and/or forming other light extraction features on the surface of the N-type GaN 14.
Forming thick layers of N-type GaN 14 via epitaxial growth is operationally difficult, time consuming, and expensive. For example, the relatively thick N-type GaN 14 may release stress during cool down of an epitaxial process via formation of cracks in the epitaxial N-type GaN 14. Typically, the substrate 12 includes silicon (Si), sapphire (Al2O3), silicon carbide (SiC), and/or other “non-native” materials because “native” materials (e.g., GaN or InGaN) with usable dimensions are difficult to produce. The non-native materials typically have coefficients of thermal expansion (“CTE”) different than that of the N-type GaN 14. Such CTE mismatch may result in thermal stress causing warpage in the substrate 12 and/or other crystal defects in the N-type GaN 14 during epitaxial growth.